Hello: I was trying to install the proportioning valve back on the sub frame tonight, but it seems the front stainless steel line is to short. The assembly manual shows the valve mounting close to the end of the sub frame. The holes fro mounting are their but the front line won't reach. Has anyone else had this problem? The car is 01D, might that make a difference or are all cars the same? I would appreciate any help. Always something.....Thks.:Sam

I disasembled this part for rebuild. It contains a valve seat, 2 seals, a poppet valve stem and a spring. This configuration of parts will allow fluid (or air) to pass only when a certain presure (that of the spring) is exceded. Call it what ever you like.... I appreciate all the help and photo's.

It's a proportioning valve that progressively limits pressure to the rear drum brakes based on line pressure at the inlet; the round valve under the master cylinder is a metering valve (or "hold-off" valve), which doesn't allow fluid pressure to the front discs until there's 30-40 psi in the line. The distribution block has no valving in it at all - it only activates its internal differential pressure warning switch (which lights the warning lamp in the cluster) when it senses a pressure differential between the front and rear systems.

I can't explain why Chevy named them the way they did, but functionally, the one on the frame is a true proportioning valve (and its function was later incorporated into the "combination valve" that combined proportioning, front metering, distribution, and differential pressure switch functions in the mid-70's). The round one under the master cylinder has no proportioning function - it just remains closed until its inlet sees 30-40 psi, then it opens and allows fluid pressure to the front discs.

The function of the round metering valve (or "hold-off" valve) is to allow the rear system to see 30-40 psi (which moves the rear drum shoes out into contact with the drums) before the front discs see any pressure, to avoid disconcerting "front brakes first" on pedal application, which causes "nose-dive" at low speed. The metering valve on my '69 Z/28 is stuck open (needs a rebuild), and at low speed light brake application the car nose-dives and feels like it'll stand straight up on its nose when you step on the brakes. Gotta rebuild it this winter if I can find the correct kit with ALL the seals.

Does anyone know of a source for the soft goods for owner rebuild of the subframe mounted:"pressure regulating valve" or "proportioning valve" by whatever name,,,1968 disc brake car w/ drivers side subframe mounted in-line pressure <do-hickey>

I have unmounted mine and removed the hex head cap screw and 1 rubber "V"-grooved cup seal and washer, to the point of a external snap ring and am not sure what lies beneath,,,,Lots of 'gunk' so I was interested in further disassy and cleaning, just was checking if others have dug into this component and sources for the soft goods needed to PROPERLY rebuild it, otherwise its just a careful cleaning and reassembly,,,

I can't explain why Chevy named them the way they did, but functionally, the one on the frame is a true proportioning valve (and its function was later incorporated into the "combination valve" that combined proportioning, front metering, distribution, and differential pressure switch functions in the mid-70's). The round one under the master cylinder has no proportioning function - it just remains closed until its inlet sees 30-40 psi, then it opens and allows fluid pressure to the front discs.

The function of the round metering valve (or "hold-off" valve) is to allow the rear system to see 30-40 psi (which moves the rear drum shoes out into contact with the drums) before the front discs see any pressure, to avoid disconcerting "front brakes first" on pedal application, which causes "nose-dive" at low speed. The metering valve on my '69 Z/28 is stuck open (needs a rebuild), and at low speed light brake application the car nose-dives and feels like it'll stand straight up on its nose when you step on the brakes. Gotta rebuild it this winter if I can find the correct kit with ALL the seals.

With that said I think my hold off valve (round valve) may not be opening soon enough since I tend to get premature rear brake lock up during panic stops. Because of this I took apart the subframe mounted proportioning valve and cleaned out what appeared to be hardened or glazed brake fluid gunk but the back wheels still tend to lock up during hard braking. Do you think tearing into the hold off valve will resolve my issue? how do you test the hold off valve for proper function?

As far as a supplier for the frame valve parts I had no luck at finding one .So I purchased a new re-pop from Inline tube and stole the parts out of it to rebuild mine .A little expensive but it worked .I got to keep my correctly dated valve .

The function of the round metering valve (or "hold-off" valve) is to allow the rear system to see 30-40 psi (which moves the rear drum shoes out into contact with the drums) before the front discs see any pressure, to avoid disconcerting "front brakes first" on pedal application, which causes "nose-dive" at low speed.

Ok, this is not rockets science we are dealing with here ... But I have been having nothing but problems with my brakes now that I'm near the end of my rebuild. I don't want to bore you all with all the details of my woes ... But will just start off where I am now.

So basically the entire brake system is all spanking new. There are no leaks at all and the brakes - front and rear bleed with a smooth clear stream (over and over again) and looks fine - no air. When the brake pedal is hit, the rear brakes work perfectly. No issues. They grab tight and hold.

Now the %^&#&% Front disc is another issue! They don't seem to grab at all. But yet, if I pump the brake pedal 5 or 6 times, the grab a little more with each pump, and then hold tight. Once I let off the brake, the it's the same thing again. 5-6 pumps before the front hold tight. Again the rear are working as designed.

I'm stumped and at wit's end on this. This should be a pretty simple thing and it is beating the crap out of me. I'm about ready to swap out the Master ... Again!

But after reading through this thread (good stuff), I am now thinking this just might a proportioning valve issue - even though I all ready replaced my original one (the round one under the master cylinder). Any and all help/advice would be most appreciated.

This should be in the Maintenance section. But if the piston is not returning in the master it will not function properly. And the round metering valve still may be at fault. Once again-- new does not mean good.

Ok, this is not rockets science we are dealing with here ... But I have been having nothing but problems with my brakes now that I'm near the end of my rebuild. I don't want to bore you all with all the details of my woes ... But will just start off where I am now.

So basically the entire brake system is all spanking new. There are no leaks at all and the brakes - front and rear bleed with a smooth clear stream (over and over again) and looks fine - no air. When the brake pedal is hit, the rear brakes work perfectly. No issues. They grab tight and hold.

Now the %^&#&% Front disc is another issue! They don't seem to grab at all. But yet, if I pump the brake pedal 5 or 6 times, the grab a little more with each pump, and then hold tight. Once I let off the brake, the it's the same thing again. 5-6 pumps before the front hold tight. Again the rear are working as designed.

I'm stumped and at wit's end on this. This should be a pretty simple thing and it is beating the crap out of me. I'm about ready to swap out the Master ... Again!

But after reading through this thread (good stuff), I am now thinking this just might a proportioning valve issue - even though I all ready replaced my original one (the round one under the master cylinder). Any and all help/advice would be most appreciated.

Compare old master cylinder with new, Check the depth of the hole of the master cylinder piston that the pedal pushrod fits into. Theres a chance that the piston is not being forced in far enough. I ran into this problem helping Ken Boje with the Gibb z/28

Are the calipers installed corectly? I recall reading that they will install on the wrong side, but if that is done they do not bleed correctly because of the position of the bleeder leaves air in the caliper.